This invention relates in general to radiation devices and in particular to a new and useful radiation source for infrared radiation.
The invention relates particularly to a radiation source for infrared radiation with an area radiator which is applied on a support substrate, and with an electrically conductive resistance layer which is applied as heating element on the support substrate.
From U.S. Pat. No. 3,875,413 and infrared radiator is known where the radiation source is formed as a thin film on a support. The film is used as a radiator directly as a heating resistance. The support consists of a material of low thermal conductivity, such as quartz or sapphire. In the central region of the support 11 an area type radiation source 13 is arranged, which is vaporized with a material of high electric resistance and high optical emission capacity and forms the heating resistor. It is at the same time the radiation source. The end pieces of the vaporized radiation source, contiguous in longitudinal directions are covered with contact electrodes of platinum which carry the terminal wires and cover the edges of the heating resistor. Due to the low emission capacity of platinum, the contact electrodes do not contribute to the radiation; the radiation is limited to the area of the heating resistor.
An important area of use of such infrared radiation sources is in gas analyzers, which, utilizing the infrared absorption, are able to distinguish and to measure the various components of a gas mixture to be analyzed.
In optical gas analysis, a modulated beam is employed, to permit a simple separation of the modulated wanted signal from the low-frequency random noise. If continuously luminous radiation sources are used, the radiation must be modulated by a mechanical chopper, such as a rotating mirror or rotating diaphragms. Such mechanical solutions are expensive and must be matched exactly, and because of their delicate construction they are not suitable for portable gas analyzers. The desideratum therefore is to realize the modulation of the optical beam by direct modulation of the radiation source intensity. An essential prerequisite for this is, however, that the radiation source must have as low as possible a heat capacity and that the heat removal to the surroundings takes place quickly.
The known infrared radiation sources in the form of thin metal layers on ceramic substrates have too high a heat capacity to permit a sufficiently high modulation frequency of the radiation source itself. While the known radiator needs little energy for heating because of its small total mass and the low thermal conductivity of the support, it is not suitable for radiation modulation by pulsed heating, for example, because during the heating intervals the heat is removed slowly from the radiation zone. Modulation of the radiation therefore requires an additional mechanical modulator disposed in the ray path afterwards.